Co-ordination chemistry of 8-methyl-, 8-ethyl-, and 8-isopropylquinoline-2-carboxaldehyde-N-methylimine with palladium, rhodium, and iridium. Crystal and molecular structure of (η-allyl)(8-isopropylquinoline-2-carboxaldehyde-N-methylimine-NN′)palladium(II) perchlorate

Abstract

The co-ordination chemistry of the new potentially bidentate ligands 8-methyl-, 8-ethyl-, and 8-isopropylquinoline-2-carboxaldehyde-N-methylimine (mqa, eqa, and iqa respectively), 8-RC₉H₅N–CHNMe, with palladium, rhodium, and iridium is reported. Hydrogen-1 and ¹³C n.m.r. spectra are used to show that there is unidentate co-ordination through the CHNMe nitrogen atom in the complexes [RhCl(C₈H₁₂)(mqa)], [Pd(O₂CMe)₂(mqa)₂], and [PdCl(C₆H₄CH₂NMe₂-2)(mqa)] with the 3-H of the quinolyl group positioned above the co-ordination plane. Bidentate co-ordination is found in the complexes [RhCl(CO)₂L] and [Pd(η-allyl)L]X(allyl = allyl or 2-methylallyl; X = ClO₄ or BF₄; L = mqa, eqa, or iqa). The crystal and molecular structure of [Pd(η-C₃H₅)(iqa)][ClO₄] has been determined by X-ray methods. Crystals are monoclinic, a= 8.598(1), b= 8.142(1), c= 27.400(3)Å, β= 97.151 (1)°, space group P2₁/c, and Z= 4. The structure has been solved by the heavy-atom method and refined by full-matrix least squares to R= 0.041 for 2 437 diffractometer data. The η-allyl group is essentially normal but slightly asymmetric in a way consistent with the CHNMe having a greater trans influence than the quinoline group. The major feature is the relief of an unacceptably close approach of the Prⁱ group to the Pd atom, resulting largely from the metal atom being 1.09 Å out of the C₅N plane of the quinoline ligand. Carbon–hydrogen cleavage (oxidative addition) at the 3-position of the quinolyl group leads to the hydrido-complexes [MH(Cl)(mqa – H³){P(C₆H₁₁)₃}₂](M = Rh or Ir), whereas palladation leads to metallation at different positions. The particular site of palladation may be controlled by the other ligands and it is argued that uni- and bi-dentate co-ordination lead to palladation at the 3-position and at the 8-Me group respectively. The sites of metallation have been determined from ¹H and ¹³C n.m.r spectra of soluble derivatives.